Spinal manipulation (SM) is widely used by chiropractors, osteopaths and physical therapists to treat musculoskeletal disorders of the spine. Clinical studies, however, have shown mixed results regarding the effectiveness of spinal manipulation to treat low back pain and have rarely quantified or controlled the delivery of the manipulative procedures. SM generally involves the application of manual loads to specific locations of the spine with the intent to induce intersegmental motions and stretch or compress ligaments, discs, and muscles. These anatomical structures contain sensory receptors which respond to changes in mechanical and chemical conditions and provide input to the central nervous system. There is evidence suggesting that mechanically stimulated neural responses, in particular, are important elements of the mechanism of action of SM. The National Center for Complementary and Alternative Medicine (NCCAM) has identified the need to biomechanically characterize manipulation procedures as an important area of research. Two broad categories of SM exist: high velocity low amplitude (HVLA) procedures, and low velocity variable amplitude (LWA) procedures. HVLA procedures deliver forces in a quick thrust with loading durations on the order of 100-200 milliseconds with approximately 200-500 N force. LWA procedures deliver a combination of regional spine loads and forces at localized vertebral levels delivered at a slow rate of loading and applied cyclically over a long duration on the order of 4-20 seconds with forces ranging from 80 N to 200 N. In addition to differences in temporal characteristics and loads, SM procedures vary in the point of application of these loads, and the directions along which loads are applied. Such variations may have clinical repercussions if mechanically sensitive tissues are affected in different ways. Our long term goal is to contribute to the understanding of SM mechanisms by characterizing the responses of intervertebral connective tissues to the full range of manipulative forces to provide more details about how specific SM characteristics affect the mechanical environment of spinal tissues. The objective of our proposed study is to understand the biomechanical effects of different manipulations. We propose a fouryear developmental study with experimental work at Palmer College of Chiropractic on human cadaver spines to measure directly the vertebral motions and the biomechanical strains of facet joint capsules during simulated HVLA SM and LWA SM. Co-Leaders at Stony Brook and Co-Investigators at University of Toledo will develop and validate a finite element model and estimate the strains of internal ligaments that cannot be measured. The validated computer model will be used to study the responses under varying SM load parameters.